1. Field of the Invention
The present invention relates to a space-diversity reception system, more particularly to a method and apparatus for space-diversity reception in microwave band communication using 16- or 64-quadrature amplitude modulation of digital microwaves. The present invention is advantageously used in transhorizon communication.
2. Description of the Prior Art
Space-diversity reception system are widely used in transhorizon microwave radio communication as they effectively reduce the effect of fading and noise due to fluctuations of electric fields, transmission distortion, and poor atmospheric conditions.
As well known by persons skilled in the art, for example, from Japanese Unexamined Patent Publication (Kokai) No. 56-72548, a space-diversity reception system comprises two parabolic antennas for receiving microwaves and reception systems for receiving the microwaves from these antennas. The microwaves received from the two antennas are combined to obtain an intermediate frequency output by the minimum amplitude deviation composition method. Part of the microwaves is also split off for control purposes.
The prior art method, however, requires the use of numerous narrow band-pass filters, for example, crystal filters or surface acoustic wave (SAW) filters, and detectors for filtering and detecting the predetermined intermediate frequency. Such narrow band-pass filters, however, are very expensive. Further, since the intermediate frequency itself and the desired bandwidth differ with each system, it is necessary to design and utilize different filters for each system, which is very troublesome.
Even with numerous narow band-pass filters, there is a problem of reduced performance of the space-diversity reception system when the fading occurs at frequencies other than those of the band-pass filters.